A nuclear magnetic resonance study : the kinetics of hydrogen exchange reactions in aqueous 15NH+4 solution

Thompson, Linda B. (1976) A nuclear magnetic resonance study : the kinetics of hydrogen exchange reactions in aqueous 15NH+4 solution. Masters thesis, Memorial University of Newfoundland.

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Abstract

A study of the proton exchange reactions of ¹⁵NH⁺₄ that take place in aqueous acidic solution was conducted. Nuclear magnetic resonance methods were used to measure NH and OH line broadening resulting from exchange over the temperature range 30ﾟC to 80ﾟC, enabling computation of activation parameters for the reactions: -- reaction (4): [special characters omitted] -- reaction (6): [special characters omitted], and -- reaction (7): [special characters omitted] -- Activation energies of essentially zero for reactions (-4) and (7) have been reported in a previous study of aqueous acidic ¹⁴NH⁺₄ proton exchange reactions. Since, for the recombination reaction, any mechanism which might be associated with an activation energy of zero is inconsistant with the diffusion mechanism indicated by the bulk of data on such reactions, this study was undertaken. The ¹⁵N isotope was used to eliminate the possibility of a contribution to the NH linewidth by broadening due to quadrupolar relaxation. -- The activation enthalpy for reaction (4) was found to be 14.6 kcal mole⁻¹. Rate constants for reaction (-4) were found to conform well to a model for diffusion controlled reactions with ∆H*₋₄ equal to 2.2 ± 0.3 kcal mole⁻¹, and with Po a constant equal to (4.2 ± 0.2) x 10⁻⁸ cm. ∆H*₇ was also found to have a non zero value, 4.3 ± 1.6 kcal mole⁻¹; and ∆H*₆ was found to equal 5.6 ± 1.3 kcal mole⁻¹.

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